Main Principles.
The control of all modern aeroplanes is effected in a lateral direction by small planes hinged to the rear spar of the outer ends of the wings, and known as “ailerons”; in a longitudinal or “fore-and-aft” direction by the elevator planes; and for steering by the rudder. Although these functions are alluded to separately, they are more often than not combined in their actions. The correct proportion of the controlling surfaces is an important factor in determining the ease or otherwise with which a machine can be handled in flight, and faults in this direction are responsible for the terms “heavy” or “stiff” on the controls being applied to a machine. The use of subsidiary flaps or ailerons for lateral control is a comparatively modern innovation. At one time it was usual to warp the entire plane, or in some cases the outer section only, and although the principle is the same—that of forming a negative or positive surface to the line of flight—structural considerations are wholly in favour of ailerons. With warping, the whole plane is subjected to continuous torsional movement, and to obtain this some of the trussing wires have necessarily to be arranged as control wires, the result being that the plane curvature loses its uniformity, and the whole girder system of the planes is less efficient under load than if the wires were permanently fixed; and the latter item is only possible with aileron control. Although it is usual to attach ailerons to both top and bottom planes of a biplane, there are occasions when sufficient control can be obtained with ailerons to the upper plane only, usually when the span of this plane is greater than that of the bottom.